Devices which take advantage of the energy in falling water have existed for several centuries. The earliest example is the water wheel. It is an impulse device which uses a multitude of buckets or paddles to catch falling or moving water, depositing it at a downstream point. The movement of the buckets or paddles spins a central supporting shaft which can do work or be used to generate power.
In the late 1800's and early 1900's hydro power innovators discovered the advantages of reaction turbines, which mainly generate power from pressure and energy differences in stream flow. These turbines extract more power from streams than water wheels, rotate at higher speeds and accept much larger water flow volumes. Reaction turbines, such as the Francis or Kaplan, are therefore almost exclusively used at high flow hydroelectric sites. Both of these designs utilize a rotating disc of blades oriented in a plane perpendicular to the direction of exit flow. Impulse hydro power generating devices are still used in high head (over 20 m) low flow applications (the Pelton design) and low head (3 m to 10 m) low flow applications (Cross-flow design).
The Francis, Kaplan, Pelton and Cross-flow hydro power designs are mature technologies and very well understood. None of these designs, however, is recommended for high-flow applications of less then 3 meters of head. The impulse designs cannot accept very large water flows while the Francis and Kaplan turbines are not practical. To utilize these large reaction turbines at low-head sites they must be partially elevated resulting in very low water pressures. Cavitation can result damaging the turbines and water transport surfaces. Variations on the Kaplan, such as the Starflow, inclined Kaplan and Bulb turbines, all of which are similar to water flowing through a propeller, have been recommended for low head situations, however, they are not economical for heads of less than 3 m and many rivers with higher heads and lesser flows.
One solution for economically generating power from ultra-low head hydro sites (less than 3 m of head) is adoption of the Darrieus turbine design. The Darrieus turbine was developed in France by Georges Darrieus in the 1920's to generate power from wind. It consists of a set of long, rectangular airfoils connected to a central rotating shaft. The airfoils may be curved to directly connect to the shaft or be straight and held parallel to the shaft by struts, arms or discs. These blades are thereby oriented transversely to the fluid flow and parallel to the axis of rotation. Significant testing in the 1980's and 1990's demonstrated the utility of this turbine design. Darrieus turbines were, however, not widely adopted for wind power as pinwheel type wind turbines were more economical.
In the early 1980's a Canadian innovator, Barry V. Davis, applied the vertical-axis Darrieus design to water. Several different models were successfully tested in the laboratory and various waterways. Government funding ended in the late 1980's, however, several organizations are presently attempting to utilize the Davis turbine design in ultra-low-head hydro and tidal applications.